ABSTRACT Mammals explore the chemical world with two distinct olfactory modalities, known as the main and accessory olfactory systems. The accessory olfactory system (AOS) is at least partially specialized to detect cues, often called pheromones, which regulate social behavior among members of the same species. While many insect pheromones have been identified, the understanding of the nature of these cues in mice and other mammals is still rudimentary. One powerful strategy for identifying mammalian chemical social cues is to record from the sensory neurons of the AOS, and use their spiking responses to guide the purification of active compounds from natural sources such as urine. This approach recently led to our identification of sulfated steroids as the predominant source of activity in female mouse urine. Because steroids control mammalian physiology, their metabolites represent an attractive class of social cue, revealing much about the physiological state of other members of the group. This application proposes experiments to learn more about mammalian pheromones and the neurobiology of their detection. (1) Preliminary evidence suggests many uncharacterized steroid derivatives present in urine activate sensory neurons; chief among these are compounds that appear to be derivatives of estrogen, potentially signaling information about reproductive state. By combining biochemical purification, electrophysiology, and chemical analysis, some of these other compounds will be identified. (2) Individual sulfated steroids in urine vary in their level depending on an animal's physiological state. By recording the responses of the sensory neurons to a concentration series of different sulfated steroids, the mechanisms for encoding identity and concentration will be elucidated. These experiments will also reveal whether responsive neurons fall into affinity distinct classes. (3) The detailed understanding of steroid biology facilitates the making of hypotheses about the likely behavioral role of particular sulfated steroid pheromones. Behavioral experiments probing alarm/stress, dominance, and attractiveness will reveal the precise function of individual compounds in organizing social behavior.